The present invention relates to a composition capable of being applied to paper at high speeds in a cast-coating and to the resultant cast-coated paper.
Various methods are known for manufacturing cast-coated paper, such as wet cast coating, gel cast coating, and rewet cast coating techniques. The wet cast coating method comprises applying a water-based paint containing a pigment and an adhesive to a base paper and bringing the still wet paper into contact with a high temperature heated mirror drum ("cast drum") to dry coated paper. The gel cast coating method comprises either applying a water-based paint containing a coagulant to a base still wet paper and bringing the paper into contact with the cast drum to dry the same, or else applying a water-based paint to a base paper, applying a coagulant containing coating onto the painted paper, and then bringing the still wet paper into contact with a cast drum to dry the same. The rewet cast coating process comprises the steps of applying a waterbased paint to a base paper, drying the paper and again wetting the paper prior to bringing it into contact with the cast drum to dry it. High gloss art paper is supercalendered in a dry state, while the cast-coated paper s smoothed while it is still wet, state in i.e., while the coated layer is still in a highly plastic state, in either of these cast coating techniques. The cast coating technique produces a high gloss smooth coated surface.
It is of primary importance in the cast coating process that the coated paper is clearly released from the surface of the cast drum after the coated surface containing a great deal of water is brought into contact with the cast drum to dry the same. This process is used to obtain a coated paper having a high gloss which is free of defects. It has been known that good adhesion to the cast drum surface is an essential requirement for imparting high gloss to the cast-coated paper and leads to a substantial reduction in the releasability of the coating. Therefore, manufacture of high gloss, defect-free (e.g., pin holes in the coating) cast-coated paper requires advanced technology. However, as the speed of releasing the coated paper from the cast drum is increased resistance is encountered during the releasing operation. As a result, the coated film on the paper adheres to the surface of the cast drum and a partial pick-up of the paper is sometimes observed (such defects in the coated paper being referred to as "pit(s)"). Accordingly, it is very difficult to obtain a defect-free coated paper having a uniform gloss.
Moreover, in general the heated drum is required to quickly evaporate moisture from the coated paper; but, when the machine is operated at high speed, the length of time during which the coated paper contacts on the drum is reduced. Therefore, extreme drying conditions must be employed to increase the mobility of vapor. As a result, a novel problem arises. In the wet cast coating process the paint boils, thereby destroying the coated layer. In the gel cast coating process, it is difficult to transfer a large amount of water in the gelled coated layer to the base paper, which in turn hinders formation of a uniform surface. In the rewet cast coating process, once dried, the coated layer is rewetted to impart plasticity thereto; therefore, a relatively high pressure s applied to the coated paper on the drum as compared to the foregoing two cast coating techniques. As a result, the small water pools often form between the drum surface and the coated film, which are quite difficult to remove through voids from the coated film. This leads to the formation of pits differing from those formed due to pick-up and results in a cast-coated paper having numerous defects.
In order to solve these problems associated with the cast-coated paper and to enable high speed production while maintaining the quality of the coated paper surface, a method has been proposed in which a coagulant, such as a sulfate, nitrate, formate, or acetate of a metal such as zinc, aluminum, or magnesium, is incorporated into a paint principally composed of a pigment and an adhesive such as a latex containing a polymer having repeated units derived from an unsaturated carboxylic acid as a functional monomer or casein (see Japanese Patent Publication No. 60-146097). This patent states that metal ions of the coagulant affect the carboxyl groups of the copolymeric latex to cause coagulation during drying of the coated layer, producing a porous coated layer which is more easily permeated by evaporated moisture when the coated paper is rewetted and pressed against the heated cast drum according to the rewet cast coating technique. However, this method produces a cast-coated paper having a low surface strength and the production speed is still unsatisfactory.